1. Field of the Invention
The present invention relates generally to x-ray systems, devices, and related components. More particularly, exemplary embodiments of the invention concern devices and methods that enhance x-ray flux uniformity and thus contribute to; an improved signal-to-noise ratio and increased dynamic range in the x-ray imaging device.
2. Related Technology
The ability to consistently develop high quality radiographic images is an important element in the usefulness and effectiveness of x-ray imaging devices as diagnostic tools. However, various problems and shortcomings relating to the design, construction and/or operation of the x-ray device often act to materially compromise the quality of radiographic images generated by the device. One problem commonly encountered in x-ray devices is the occurrence of undesirable variation in the intensity, or flux, of x-rays produced by the target. Such variations in x-ray intensity often cause visible differences in the image density of the radiographs, thereby impairing the quality and usefulness of the image. As discussed below, this lack of flux uniformity is due at least in part to anode geometry and other related considerations.
In typical x-ray tubes, x-rays are produced when an electron beam generated by the cathode is directed to a target surface or a target track, composed of a refractory metal such as tungsten, of an associated anode. In many instances, the electron beam penetrates the target surface. Such penetration of the target surface usually occurs when the target surface is worn and/or has other irregularities, but can occur under other circumstances as well.
In general, when x-rays are generated below the target surface, such x-rays typically take a variety of different paths through the target material to the x-ray subject. Because some of such paths are relatively longer than others, the anode material imparts a filtering effect to, or attenuates, the generated x-rays and so that the photon fluence and the spectral distribution are thereby affected. This phenomenon is sometimes referred to as the “heel effect.”
One particular consequence of the heel effect with respect to the x-ray beam is that the mean energy of the x-ray spectrum is relatively higher in some areas of the x-ray beam than in others. While this effect is cause for concern in a variety of different type of x-ray tube configurations, the heel effect is particularly acute in rotating anode type tubes since the targets employed in such tubes have relatively small angles, some as low as about 7 degrees. Cone beam computed tomography (“CBCT”) devices and processes are particularly susceptible.
As suggested above, the anode geometry, and the geometry of the target track in particular, plays a role in producing the heel effect whereby x-rays that are required to travel relatively further through the target track will experience a relatively greater degree of attenuation than x-rays traveling a relatively shorter distance through the target track. More particularly, the distance traveled by the x-ray through the target track is largely a function of the takeoff angle of the x-ray, or the angle of the travel path of the emitted x-ray with respect to a reference axis, such as an axis parallel to the target surface. Thus, a relatively smaller takeoff angle corresponds to a relatively shorter distance for the x-ray to travel through the target track, while a relatively larger takeoff angle corresponds to a relatively longer distance traveled through the target track material. This relationship, and the relative magnitude of the resulting effects, can be considered in terms of the relation of the takeoff angle of the x-ray to the track angle of the anode.
In particular, as the takeoff angle approaches the track angle, the travel path of the x-ray moves closer to a parallel orientation with respect to the target surface. Consequently, the degree of attenuation experienced by any particular x-ray increases as the takeoff angle of the x-ray approaches the track angle. This is readily illustrated by consideration of the end conditions where an x-ray travels either parallel or perpendicular to the target surface. In particular, an x-ray traveling parallel to the target surface travels a greater distance through the target material than an x-ray traveling perpendicular to the target surface.
Such variations in attenuation imposed on the x-rays by the target track material results in a lack of flux uniformity in the x-ray beam. It is often the case that the flux, or intensity is relatively, higher at the center of the x-ray beam and relatively lower along the edges or peripheral portions of the x-ray beam. While irregularities in flux uniformity are often attributable to considerations such as the anode geometry and the condition of the anode, flux variations may be a function of other variables as well. For example, the distance between the x-ray beam source and the imaging plane may also play a role in the relative uniformity of the flux associated with an x-ray device.
It was noted earlier that a lack of uniform flux in the x-ray beam implicates a variety of different problems. For example, nonuniform flux contributes to unacceptably high signal-to-noise ratios (“SNR”). In particular, the signal, or usable portion, of the x-ray beam is smaller relative to the noise, or unusable portion, of the x-ray beam, than might otherwise be the case. Thus, the portion of the x-ray beam that can be effectively employed in radiographic imaging processes is reduced.
Another concern relates to the impact that nonuniform flux has with respect to a dynamic range of an imager. In particular, to the extent that the flux varies over the imager, the dose available to the edges of the detectors is reduced relative to the dose available elsewhere and, thus, the dynamic range of the imager is correspondingly impaired.
In recognition of these, and other problems, attempts have been made to overcome the problems flowing from the influence of the heel effect. One such attempt involves the calibration of a flat panel imager. Generally, this attempt is a software implemented approach that involves exposing the flat panel imager to an x-ray flux and compensating gains for each pixel based upon a combination of the dose to, and response of, each pixel. If a dose to a particular pixel is reduced, the gain for that pixel is increased. By performing this process repeatedly, the gain of the unattenuated x-ray beam can be flattened somewhat.
This calibration process thus represents somewhat of an after-the-fact approach to nonuniform flux. In particular, this approach concentrates on modifying a response of the imager to the unattenuated x-ray beam, rather than performing any attenuation process on the x-ray beam itself.
The flat panel imager calibration process is largely directed to calibration of imager gain, but does little or nothing to reduce the overall dynamic gain of the x-ray system. Further, the calibration process can be time consuming.
In view of the foregoing, and other, problems in the art, it would be useful to provide methods and devices that, among other things, implement selective attenuation of an x-ray beam so as to aid in overcoming the heel effect, and other phenomena, and thus contribute to a relative improvement in flux uniformity of the x-ray beam.